With ever-increasing demand for portable computers, cellular telephones, and portable electronic devices of all kinds, the need for improved rechargeable batteries has correspondingly also increased. Rechargeable batteries, also known as "secondary batteries", consist basically of a cathode, an anode, and a liquid electrolyte or other material interposed between the cathode and anode, and which allows the movement of ions between the anode and cathode. The properties sought in rechargeable batteries include high capacity, good cyclability, and high voltage durability.
The "capacity" of a rechargeable battery may be measured in units of mAh/g and is a measure of deliverable charge per unit weight of material. The "cyclability" of a rechargeable battery, also known as "cycling behavior" or "cycle life", is a measure of the rate at which the battery's capacity decreases, or "fades", over a course of discharge-recharge cycles. Cyclability may be represented as the average percentage reduction in capacity over the selected course of discharge-recharge cycles. High cyclability is a direct measure of the usable life of the battery and is a particularly important property when the production and resultant consumer costs of the battery are high. The "voltage durability" of a rechargeable battery is typically measured in units of volts and represents the upper limit of voltage to which a battery can be charged without causing damage to the structural integrity of the cathode material. Therefore, a battery having a relatively high voltage durability will perform as well in terms of capacity fade when charged to high voltages as it would when charged to lower voltages. Greater voltage durability is desirable because batteries charged to higher voltages can yield higher capacities and energy densities. The "energy density" of a battery is the net energy that a battery can deliver during discharge per unit weight of the active cathode material. Energy density is an integrated product of the voltage and charge per unit weight of the material and may be calculated as E = (1/M).sub.o .intg.V(q)dq, where M is the weight of the active material in the cathode, Q is the net charge after discharge, and V(q) is voltage as a function of charge (q). A high energy density is desirable because a battery of a given weight having a higher energy density could deliver higher energy for a given application.
The accelerating commercial significance of the market for rechargeable batteries renders an improvement in any of the foregoing properties, a substantial advantage over the existing battery formulations. Therefore, investigators have continually sought to provide improved rechargeable battery cathode compositions.
One family of rechargeable batteries, the lithium-ion rechargeable batteries, include a cathode composed of lithium-based crystalline material. The crystal lattice of the cathode material provides a structural framework for lithium ions: lithium ions may be removed from and intercalated back into the cathode's crystalline framework. Thus, the cathode material is considered the "active" material in lithium-ion rechargeable batteries. During charging, lithium ions are removed from the cathode material and are either deposited on or intercalated into the anode (depending on the type of anode used). During discharge, the lithium ions are intercalated back into the cathode material, facilitating the flow of current between the battery's terminals. During the charge and discharge cycles, the cathode containing the active material should not undergo any significant structural change in order to preserve the reversibility of the reaction. It is the stability of the crystal structure of the cathode material that allows the reactions to be reversed and the reserve of lithium ions to be repeatedly intercalated back into the cathode material during the discharge reaction. It will be understood that the performance of the battery depends in large part on the composition of the cathode material, which in turn directly influences the battery's specific capacity, energy density, current capability, and cyclability.
The family of lithium-ion batteries include those wherein the cathode is composed of lithium transition metal oxides. Lithium transition metal oxides commonly known for use as cathode material include lithium manganese oxide-spinel (LiMn.sub.2 O.sub.4), lithium nickel oxide (LiNiO.sub.2), and lithium cobalt oxide (LiCoO.sub.2). All of these materials work by the reversible transfer of lithium ions discussed above. During the charge cycle, lithium ions are removed from the cathode and are deposited on or intercalated into the anode, leaving the intrinsic structure of the cathode substantially intact. During discharge, the lithium ions spontaneously intercalate back into the cathode. The common lithium transition metal oxides show a high voltage with respect to lithium (in the range of 3.8 to 4 V) and, because they have low molecular weights, provide a high energy density.
Table 1 provides the energy density of LiMn.sub.2 O.sub.4, LiNiO.sub.2, and LiCoO.sub.2 cathode materials using coke as anodes according to K. Brandt, Solid State Ionics, 69 (1994), pages 173-183. Table 1 also ranks the relative cyclability, cost of manufacture, and ease of synthesis of the materials based on current information generally available to those in the battery art.
TABLE 1 ______________________________________ Energy Density Ease Material (Wh/kg) Cyclability Cost of Synthesis ______________________________________ LiCoO.sub.2 276 good high easy LiNiO.sub.2 321 poor intermediate difficult LiMn.sub.2 O.sub.4 305 intermediate low intermediate ______________________________________
Among the three materials, LiMn.sub.2 O.sub.4 and LiNiO.sub.2 currently are significantly less expensive to manufacture and have less of an environmental impact than LiCoO.sub.2. However, the inconsistent cycling behavior of LiMn.sub.2 O.sub.4 and LiNiO.sub.2 presently limits their application. If the cyclability of LiMn.sub.2 O.sub.4 and LiNiO.sub.2 is improved, it is possible that the materials could replace LiCoO.sub.2 as the preferred cathode material in lithium-ion rechargeable batteries.
One approach that has been taken to improve the cycling stability derived from lithium transition metal oxides is to include in the materials other elements in the periodic table. In some instances, the resulting variation of the crystal structure from, for example, the LiMn.sub.2 O.sub.4, LiNiO.sub.2, and LiCoO.sub.2 stoichiometries of the undoped compounds, has resulted in improvements in the materials' electrochemical properties.
R. J. Gummow et al., "Improved Capacity Retention in Rechargeable 4 V Lithium/Lithium-Manganese Oxide (Spinel) Cells", Solid State Ionics (69), pages 59-67, suggests that the addition of excess lithium in LiMn.sub.2 O.sub.4 improves the material's cycling behavior. The reference also suggests that the substitution of a portion of the manganese in LiMn.sub.2 O.sub.4 with additions of magnesium or zinc provides improved cycling behavior. However, although improvement in the materials' cycling stability may result, the specific capacity of the magnesium-or zinc-doped LiMn.sub.2 O.sub.4 spinel materials is low and in the range of 90-105 mAh/g.
U.S. Pat. No. 5,264,201, the entire disclosure of which is hereby incorporated herein by reference, discloses a material having improved cycling stability represented by the formula Li.sub.x Ni.sub.2-x-y M.sub.y O.sub.2 wherein x is between about 0.8 and about 1.0, M is one or more of cobalt, iron, titanium, manganese, chromium, and vanadium, and y.ltoreq.0.2, except that y.ltoreq.0.5 for cobalt. In pure LiNiO.sub.2, metal atom layers of substantially pure lithium and substantially pure nickel alternate between layers of substantially pure oxygen. As the stoichiometry of lithium decreases from 1.0, nickel atoms are incorporated into the lithium atom layer, which inhibits de-intercalation of lithium atoms. The inventors of the '201 patent suggest that by maintaining x within the stated range, nickel atoms are not incorporated into the lithium atom layers in amounts that substantially reduce the ability of lithium to be de-intercalated. Although the different substitutions and adjustments in stoichiometry suggested in the '201 patent may have shown some improvement over pure LiNiO.sub.2 materials, the improved materials still do not provide the cycling stability of LiCoO.sub.2 materials. Neither do the materials of the '201 patent posses the voltage durability believed necessary to significantly improve the performance and, hence, the applicability of the materials.
U.S. Pat. No. 5,591,543, the entire disclosure of which is hereby incorporated herein by reference, discloses a family of lithium-ion cathode materials of the composition Li.sub.1-x Q.sub.x/2 ZO.sub.m, wherein Z is a transition metal selected from cobalt, nickel, manganese, iron, and vanadium, Q is selected from the group II elements calcium, magnesium, strontium, and barium, and m is 2 or 2.5 depending on the identity of Z. The '543 patent's inventors note that the addition of oxides or carbonate compounds of the element Q during synthesis is believed to result in the incorporation of a portion of Q.sup.+2 cations into lithium sites within the LiCoO.sub.2 lattice, while the remainder of the QO or QCO.sub.3 compounds becomes admixed in that form within the finished cathode. The dissolution of the group II oxides and carbonates during cycling buffers the electrolyte and acts as a desiccant by reacting with water and acid impurities in the electrolyte. The inventors of the '543 patent possibly ascribe the improved cyclability of their materials to the desiccating action of the group II oxides and carbonates.
As shown by FIGS. 4, 6, and 8 of the '543 patent, secondary cells incorporating cathodes of the materials disclosed in that patent may be charged up to 4.2 V. The need remains for lithium transition metal oxide cathode materials allowing higher charging voltages. First of all, materials that can be charged to higher voltage (in excess of 4.2 V) can exhibit higher capacities and energy densities. As explained above, such improved properties may provide a significant advantage in an application. Moreover, current cathode materials exhibiting the R3m (also referenced herein as "R-3m") crystal structure are not stable when charged to voltages in excess of about 4.2 to 4.3. V. When the lithium content in the cathode materials is reduced to a certain concentration during charging (in the range of about 4.2 to 4.3 V), a structural change may occur in the materials that makes their fade characteristics poor. Thus, the ability of a cathode material to be charged above 4.2 V is an important characteristic. If a cathode material will remain stable on repeated charging up to voltages higher than those achievable in current cathode material formulations, the improved material can provide additional capacity during discharge.
Accordingly, even considering the incremental improvements in cycling stability and other electrochemical properties that may be achieved by the inventions described above and other recent advances in secondary battery construction, a need remains for lithium-ion cathode materials providing improved cycling stability, higher voltage capacities, and corresponding improvements in other related electrochemical properties.